The invention concerns a rotary intaglio printing machine and a rotary intaglio printing process.
A rotary intaglio printing machine, and a process for rotary intaglio printing, as described for example in German Patent 35 30 561, involve passing a web of paper to be printed upon through at least two press units, each of which comprises a printing mechanism, with a drying device arranged between the printing mechanisms, for successively applying at least two inks to one and the same side of the web of paper as it passes through the machine. In order to provide for multi-ink or multi-color printing on the web of paper, each printing ink has its own printing mechanism. The main components of each such printing mechanism are a rotary form or plate cylinder which dips into an ink trough or fountain and against which the web of paper, as it passes through the printing mechanism, is pressed by means of a rotating impression cylinder. The plate cylinder and the co-operating impression cylinder form therebetween a gap or nip which is referred to herein as the printing nip. The web of paper is fed to the printing nip of the printing mechanism from above. The web of paper is guided and changed in its direction of run, by means of guide spindles. Downstream of each printing nip, the web of paper must pass through drying devices as, unlike the situation in offset printing, the rotary intaglio printing process provides that the freshly applied ink is very substantially dried before the next ink is applied to the web of paper.
The drying devices may be formed by drying cylinders, as in German Patent 35 30 561, or by downwardly open, box-like dryer housings which are disposed above each printing mechanism and into which the web of paper issuing from the printing mechanism passes. In the dryer housing the web of paper is guided over a distance which is as long as possible, by means of guide and direction-changing rollers, which are referred to as guide spindles, before the web of paper passes from above to the next printing mechanism in order to be printed upon therein with a further ink.
In order to carry out the drying operation, provided in each of the dryer housings are so-called impingement jet nozzle arrangements which are disposed in respective pairs on mutually opposite sides of the web of paper. Each of those impingement jet nozzle arrangements includes a plurality of nozzle pipes which are parallel to each other and which are arranged in succession in the direction of movement of the web of paper, in such a way that they extend transversely to the direction of movement, substantially over the entire width of the web of paper. Each of the nozzle pipes has a plurality of air discharge nozzles with which the nozzle pipe faces the respective side of the web of paper at a small spacing therefrom. A specific fan is provided for each impingement jet nozzle arrangement, the fan sucking air out of the interior of the dryer housing and feeding it to the nozzle pipes, the air issuing in a jet through the nozzle openings of the nozzle pipes and impinging on the respective side of the web of paper. As that air is heated, in results in drying of the ink applied in the preceding printing mechanism, that is to say the solvent in the ink, for example toluene, is caused to diffuse extensively out of the application of ink and the paper. As a result the air which is circulated in the dryer housing by the fan has an increased content of solvent vapours. In order to prevent the concentration of solvent vapours from rising above an admissible value (generally 50% of the lower explosion limit), air must be continuously sucked away out of the dryer housing and fed to a solvent separation installation.
The solvent separation installation operates all the better, the greater the degree to which the air supplied thereto is charged with solvent vapours. The optimum procedure therefore would involve going as closely as possible to the above-mentioned admissible limit value. That is not possible in practice however because another purpose of sucking air out of the dryer housing is to generate a reduced pressure which is intended as far as possible to prevent solvent vapours from being blown out of the dryer housing into the area immediately surrounding it. Because of the great width of the openings which the dryer housing has, in particular towards the printing mechanism which is disposed therebeneath, for the last-mentioned purpose air must be sucked out of the dryer housing to such a great degree that the optimum level of solvent concentration is far from being attained. Nonetheless, it is not possible totally to prevent solvent vapours from issuing into the area surrounding the dryer housings and printing mechanisms, so that the entire printing machine must be covered over with a downwardly open hood from which air is further sucked so that the solvent vapours which accumulate in the hood are also fed to the separation installation. The level of solvent concentration in the air which is sucked out of the machine hood is even lower and thus more disadvantageous, than in the air which is taken from the dryer housings.
Disadvantages arise not only from the high level of energy expenditure which is required for operation of the known dryer apparatuses but also from the large number of the above mentioned guide spindles.
As for example eight printing units are required for printing in four colors on both sides of a web of material, and each of the printing units includes typically between about seven and ten guide spindles, the previous rotary printing machines suffer from a series of difficulties:
Thus for example a major problem is that none of the many guide spindles has its own drive means, but each is driven in rotation by the web of paper to be printed upon. The result of that arrangement is that a large amount of wastage is produced at each change in speed of the machine, in particular when the machine starts up and stops. More specifically, when for example the web of paper is accelerated, the guide spindles exert a retardation effect while when the web of paper slows down, the guide spindles have a tendency to continue to rotate at their previous high speed so that they first have to be retarded by the web of paper. Accordingly, when there is a change in the speed of movement of the web of paper through the machine, a period of time elapses before the newly intended stable condition is reached, in which the register condition of the web of paper can be regulated with the required degree of accuracy. All of the paper which is in the machine and which passes into the machine in such a transitional period is printed upon with an inadequate degree of register accuracy, and is therefore not suitable for use.
Particular problems arise if the machine has to be stopped very quickly for example because of a fire or because the web of paper tears. Not least because of the enormous size of the conventional rotary intaglio printing machines which are more particularly suitable for printing in four colors on both sides of a web of paper however malfunction situations of that kind are not infrequent and, depending on the printed item being produced, an amount of wastage of between 6% and 10% generally has to be tolerated.